The present invention is generally related to saboted projectiles and more particularly to armor-piercing fin-stabilized discarding sabot (APFSDS) ammunition.
There exists a well-developed art in the field of APFSDS (including, inter alia, APFSDS-T (with tracer)) ammunition. APFSDS rounds have been developed for both rifled barrels (e.g., the 105 mm barrel of the relatively old M60 tank) and smoothbore barrels (e.g., the 120 mm barrel of the relatively new M1A2 tank). A rifled barrel or tube functions to spin-stabilize a projectile, a principle utilized in a majority of modern weapons from handguns to large naval guns. A projectile exiting the muzzle of a rifled tube typically has a relatively high spin rate. This rifling-induced spin rate is nominally equal to the product of the muzzle velocity (longitudinal) and the rifling pitch (measured in turns or revolutions per linear dimension). An exemplary 105 mm rifled tube has a 1-18 twist, meaning the longitudinal distance for the rifling to make one complete revolution is eighteen times the caliber of the barrel. Thus, the exemplary pitch is one turn per 1.89 meters. With an exemplary muzzle velocity of from about 1,375 to about 1,650 meters per second, the associated spin rate will be from about 730 to about 870 revolutions per second (rps). Such a spin rate would adversely affect the performance of an APFSDS round as, once the projectile is free of the sabot, it relies on its aerodynamic fins for stability at a relatively low spin rate. The rapid angular deceleration from the rifling-induced spin rate to the preferred low spin rate may: (a) damage the projectile; (b) require a weight penalty associated with providing particularly robust fins to avoid damage; and/or (c) induce wobble or other forms of instability.
Early APFSDS rounds for rifled tubes decoupled rotation of the projectile from rotation induced by the rifling. This was done by providing the sabot with an obturator which was mounted on the sabot body in such a way as to allow the obturator to rotate about the longitudinal axis of the sabot. In such a system, the obturator engages the tube bore, accommodating to the rifling and forming a seal to retain propellant gases behind the obturator. Because of its accommodation to the rifling, the obturator acquires the rifling-induced spin rate described above. This spin rate, however, is not entirely translated to the combination of the sabot body and projectile. With standard aluminum sabots and their associated projectiles, the combination typically has a sufficient moment of inertia about the longitudinal axis to overcome the static frictional force along the annular engagement between the obturator and sabot body to allow rotation of the obturator relative to the sabot body. Thus, the sabot body and projectile spin at a rate less than the obturator. A properly designed slip obturator results in a projectile spin rate of approximately ten percent of the rifling-induced spin rate or roughly 70 rps with certain projectiles.
More recently, sabot bodies substantially formed of composite materials have been introduced, offering a significant weight reduction over their aluminum predecessors. These composite sabots further reduce weight and further increase performance from smoothbore tubes. Exemplary methods and apparatus for manufacturing such sabots are disclosed in U.S. Pat. Nos. 5,635,660 and 5,640,054, the disclosures of which are incorporated herein by reference in their entireties. Such lightweight sabots have less polar moment of inertia, making it more difficult to incorporate a slip obturator which produces a sufficiently low projectile spin rate when fired from a rifled tube.